Conventionally, a type of a semiconductor laser includes an active layer facet portion comprising a semiconductor material having a larger energy band gap than that of other regions of the active layer so that the active layer facet portion is prevented from serving as a light absorbing region due to surface levels existing at the active layer facet portion so that the active layer facet portion serves as a window layer not absorbing emitted laser light.
FIG. 13 is a perspective view illustrating a structure of a prior art ridge type semiconductor laser having a window layer partly in cross-section disclosed in, for example, Japanese Journal of Applied Physics, 30 (1991),pp. L904.about.L906. FIGS. 14(a)-14(f) and FIGS. 15(a)-15(f) are diagrams for explaining fabrication methods of these semiconductor lasers, respectively, for respective major process steps, wherein FIGS. 14(a)-14(f) illustrate structures in cross-section parallel with the resonator length direction of the laser structure (a cross-section along line 14--14 of FIG. 13), and FIGS. 15(a)-15(f) respectively illustrate structures in cross-section perpendicular to the resonator length direction of the laser structure. FIGS. 16(a) and 16(b) and FIGS. 17(a) and 17(b) are diagrams for explaining the processes for dicing the laser elements formed on the semiconductor wafers into respective chips, where FIGS. 16(a) and 16(b) illustrate a manner of cleaving the semiconductor wafer along a direction perpendicular to the resonator direction of respective laser elements, FIG. 17(a) illustrates a manner of forming a window layer on the cleaved facet, and FIG. 17(b) illustrates a manner of separating the wafer pieces obtained by the cleavage into respective chips.
In these figures, reference numeral 200 designates an n type GaAs wafer on an upper surface of which a plurality of chip regions 200a each for producing a ridge type semiconductor laser chip having a window layer are formed, and reference numeral 201 designates ridge type semiconductor laser chips each having a window layer and emitting the laser light L, which are respectively formed on the chip regions 200a of the wafer 200 and diced from the wafer 200.
The laser chip 201 has a laminated layer structure comprising an n type AlGaAs lower cladding layer 2, an undoped AlGaAs active layer 3, and a p type AlGaAs upper cladding layer 4 formed on an n type GaAs substrate 1. The upper cladding layer 4 has a linear projecting portion 4a extending in the resonator length direction at a central portion thereof, and this linear projecting portion 4a and the p type GaAs cap layer 5a covering the upper surface thereof forms a ridge 211 of the laser chip 201, and n type GaAs current blocking layers 7a are formed on the upper cladding layer 4 at both sides of the ridge 211.
A p type GaAs contact layer 8 is formed on the entire surface of the current blocking layers 7a and the ridge 211, and a surface electrode 9 0.1 to 0.3 .mu.m thick comprising a Ti/Pt/Au laminated layer film is formed thereon. A rear surface electrode 10 0.1 to 0.3 .mu.m thick comprising an AuGe/Ni/Au laminated layer film is formed on the rear surface of the chip substrate 1 and an undoped AlGaAs window layer 7b is formed at the light emitting facet of the laser chip 201.
The lower cladding layer 2, the active layer 3, and the upper cladding layer 4 are, for example, about 1.5 .mu.m, 0.03 m, and 1.5 .mu.m, thick respectively, and the n type lower cladding layer 2 has an impurity (Se) concentration of about 5.times.10.sup.17 cm.sup.-3 and the p type upper cladding layer 4 has a dopant impurity (Zn) concentration of about 1.times.10.sup.18 cm.sup.-3.
The p type cap layer 5 is 0.5 .mu.m thick and has a dopant impurity (Zn) concentration of about 1.times.10.sup.19 cm.sup.-3, and the p type contact layer 8 is 1.5 .mu.m thick and has a dopant impurity (Zn) concentration of about 1.times.10.sup.19 cm.sup.-3. The current blocking layer 7a is 1.7 .mu.m thick and has a dopant impurity (Si) concentration of 2.times.10.sup.18 cm.sup.-3 and the window layer 7b is about several hundreds .ANG. long in the resonator length direction.
A description is given of the fabrication process.
First of all, an n type AlGaAs lower cladding layer 2, an undoped AlGaAs active layer 3, a p type AlGaAs upper cladding layer 4, and a p type GaAs cap layer 5 are successively grown on the n type GaAs wafer 200 to the respective above-described thicknesses by MOCVD or the like (FIG. 14(a), FIG. 15(a)).
A SiN film is deposited on the entire surface, about 0.1 .mu.m thick, by CVD method and, thereafter, the SiN film is patterned by photolithography and selective etching, whereby a SiN film 6 serving as an etching mask is formed only at a portion corresponding to the ridge 211 of each chip region 200a which is assigned on the wafer substrate 200 (FIG. 14(b), FIG. 15(b)). Subsequently, employing the stripe-shaped SiN film 6 as an etching mask, the p type cap layer 5 and the p type upper cladding layer 4 are etched to about 0.3 .mu.m height on the undoped active layer 3, thereby forming a ridge 211 (FIG. 14(c), FIG. 15(c)).
Employing the stripe-shaped SiN film 6 as a selective growth mask, an n type GaAs current blocking layer 7a is grown to about 1.7 .mu.m thick by MOCVD or the like (FIG. 14(d), FIG. 15(d)). Thereafter, the stripe-shaped SiN film 6 is removed, and the p type GaAs contact layer 8 is grown on the entire surface on the ridge 211 and the n type current blocking layers 7a by crystal growth (FIG. 14(e), FIG. 15(e)).
Then, Ti, Pt, and Au are laminated on the p type contact layer 8 to form a surface electrode 9, AuGe/Ni/Au is laminated on the rear surface of the wafer to form a rear surface electrode 10, and, thereafter, chip separation and formation of a window layer 7b are performed (FIG. 14(f), FIG. 15(f)), as described in detail below.
First of all, the wafer 200 is separated by cleavage along the direction X perpendicular to the resonator length direction to form wafer divided pieces 201a.about.201g (FIG. 16(a), 16(b)). Respective wafer divided pieces are made to stand as shown in FIG. 17(a), and an undoped AlGaAs window layer 7b is grown on the surface to be a laser facet by crystal growth. Thereafter, respective wafer divided pieces 201d having the window layer 7b are divided into respective chips by cleavage or dicing along the resonator length direction as shown in FIG. 17(b) and 17(c). Thus a ridge type semiconductor laser chip 201 having a window layer is formed.
A description is given of another fabrication process for a prior art ridge type semiconductor laser having a window layer.
FIG. 18 is a perspective view partly in cross-section illustrating a structure of a ridge type semiconductor laser having a window layer fabricated by a process different from that described above, and FIGS. 19(a)-19(f) and FIGS. 20(a)-20(f) are diagrams for explaining the fabrication process in major process steps. More particularly, FIGS. 19(a)-19(f) illustrate structures in cross section parallel to the resonator length direction in the laser structure (in cross-section along line 19--19 in FIG. 18), and FIGS. 20(a)-20(f) illustrate structures in cross section vertical to the resonator length direction in the laser structure (in cross-section along line 20--20 of FIG. 18) in respective major process steps.
In these figures, reference numeral 202 designates a ridge type semiconductor laser chip having a window layer. This laser chip 202 includes a laminated layer structure comprising an n type AlGaAs lower cladding layer 2, an undoped AlGaAs active layer 3, and a p type AlGaAs upper cladding layer 4 formed on an n type GaAs chip substrate 1, similarly as the laser chip 201 shown in FIGS. 13 to 15. The ridge 212 of the laser chip 202 includes a linear projecting portion 2a extending in the resonator length direction formed at the central portion of the lower cladding layer 2, and an undoped active layer 3a, an upper cladding layer 4a, and a p type GaAs cap layer 5a covering the surface of the upper cladding layer 4a which are successively laminated on the linear projecting portion 2a. N type AlGaAs current blocking layers 17a are formed on the lower cladding layer 2 at both sides of the ridge 212 and an n type AlGaAs window layer 17b is formed on the lower cladding layer 2 at the side of the laser emitting facet of the ridge 212.
A p type GaAs contact layer 8 is formed on the entire surface of the current blocking layers 17a and the ridge 212, and a surface electrode 9 comprising a Ti/Pt/Au laminated layer film is formed thereon. At the rear surface side of the chip substrate 1, a rear surface electrode 10 comprising an AuGe/Ni/Au laminated layer film is formed. The other construction is the same as that shown in FIGS. 13 to 15(f).
A description is given of the fabrication method.
First of all, an n type AlGaAs lower cladding layer 2, an undoped AlGaAs active layer 3, a p type AlGaAs upper cladding layer 4, and a p type GaAs cap layer 5 are successively grown on an n type GaAs wafer 200 to predetermined thicknesses by crystal growth such as MOCVD (FIG. 19(a), FIG. 20(a)).
SiN is deposited on the entire surface by CVD or the like, and the SiN film is patterned by photolithography and etching so that a portion corresponding to the ridge 212 of the laser structure remains and an aperture 16a is formed on an extension line of the ridge at the chip region end, whereby a SiN film 16 serving as an etching mask as well as a selective growth mask is formed (FIG. 19(b), FIG. 20(b)).
Employing the SiN film 16 as a mask, surface portions of the p type GaAs cap layer 5, the p type AlGaAs upper cladding layer 4, the undoped AlGaAs active layer 3, and the p type AlGaAs lower cladding layer 2 are etched to expose the side surfaces of the active layer 3, thereby forming the ridge 212 (FIG. 19(c), FIG. 20(c)).
Employing the SiN film 16 as a selective growth mask, a high resistivity AlGaAs layer 17 is grown on the n type lower cladding layer 2 exposed by etching at both sides of the ridge 212 and at the side of the light emitting facet by MOCVD or the like, whereby current blocking layers 17a are formed at both sides of the ridge 212 and a window layer 17b is formed at the side of the light emitting facet (FIG. 19(d), FIG. 20(d)).
After the SiN film 16 is removed, a p type GaAs contact layer 8 is formed over the ridge 212, the current blocking layers 17a, and the window layer 17b by crystal growth (FIG. 19(e), FIG. 20(e)), and thereafter the front surface electrode 9 and the rear surface electrode 10 are formed.
As shown in FIGS. 16(a) and 16(b), as in the prior art fabrication method described with reference to FIGS. 14(a) to 14(f) and FIGS. 15(a) to 15(f), the wafer 200 is separated by cleavage along a direction perpendicular to the resonator length direction to form a light emitting facet of the laser element of the respective chip regions 200a, and as shown in FIG. 17(c), respective wafer divided pieces 201d are cut out into respective chips by cleavage or dicing along the resonator length direction, thereby resulting in a ridge type semiconductor laser chip having a window layer 202.
In the prior art fabrication method of a semiconductor laser shown in FIGS. 14(a) to 14(f) and FIGS. 15(a) to 15(f), it is required to carry out four crystal growths, i.e., a first one for forming the n type AlGaAs lower cladding layer 2, the undoped AlGaAs active layer 3, the p type AlGaAs upper cladding layer 4, and the p type GaAs cap layer 5 on the n type GaAs wafer 200 (FIG. 14(a), FIG. 15(a)), a second one for forming the n type AlGaAs current blocking layer 7a (FIG. 14(d), FIG. 15(d)), a third one for forming the p type GaAs contact layer 8 (FIG. 14(e), FIG. 15(e)), and a fourth one for forming the undoped AlGaAs window layer 7b (FIG. 14(f), FIG. 15(f)), thereby resulting in quite inferior workability in the fabrication process. In addition, because quite a difficult process is required to be performed for growing an undoped AlGaAs window layer 7b at the laser light emitting facet formed by cleavage, it is difficult to increase production yield and the process cannot be used in mass-production.
In the other prior art fabrication method shown in FIGS. 18, FIGS. 19(a)-19(f), and FIGS. 20(a)-20(f), because the region other than the ridge formation region is etched deeper than the undoped AlGaAs active layer 3, the side surface parallel to the resonator length direction of the etched active layer 3 is exposed after the etching processing, resulting in deteriorated quality of constitutional material at the side surface of the active layer and unfavorable influence on the laser characteristics.